Introduction to Nuclear Science

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Introduction to Nuclear Science PIXIE-PAN Summer
Science Program University of Notre
Dame 2007 Tony Hyder, Professor of Physics
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Topics we will discuss Ground-state properties
of the nucleus size, shape, stability, binding
energies, angular momenta Radioactivity alpha,
beta, and gamma decay The nuclear force Nuclear
reactions the compound nucleus, Q values,
excited states Decay modes of an excited nucleus
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The radius of the nucleus
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Diffraction pattern of high-energy electrons
scattered by 16O and 12C sin? 0.61 ?/R
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• Non-spherical nuclear shapes. The electric
  quadrupole moment is given by 3z2 - x2 - y2 - z2
• gt 0 for a football on end
• 0 for a sphere
• lt 0 for an egg on the table

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Quadrupole moments of a number of odd-A nuclei.
The arrows point to spherical nuclei. The
numbers are the shell-model magic numbers about
which we will talk later.
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The curve of binding energy binding energy per
nucleon. Note that above mass 40 or so, it is
constant
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Of the 3000 or so known nuclides, there are only
266 whose ground states are stable. The rest are
radioactive
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Any idea what this is about?
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Radioactivity
For a nucleus to be radioactive at all, its mass
must be greater than the sum of the masses of the
decay products. We will look briefly at three
types of decay alpha, beta, and gamma Many of
the heavy nuclei are unstable to alpha decay, and
because the Coulomb barrier inhibits the decay
process, the half life for alpha decay can be
very long if the decay energy is small. All very
heavy nuclei (Zgt83) are theoretically unstable to
? decay since the mass of the parent is greater
than the sum of the masses of the decay products
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The Geiger-Nuttall relation
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Alpha decay The broken line is the line of
stability, i.e., the floor of the energy valley
we saw earlier
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The alpha-particle spectrum from 227Th the
highest energy alpha particles corresponds to
decay to the ground state of 223Ra
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The energy levels of 223Ra can be determined from
the measurement of the alpha-particle energies we
saw in the pervious slide. Not all of the
gamma-ray transitions are shown
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The energy spectrum of electrons emitted in beta
decay
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Beta decay
b- decay
A neutron changes into a proton and emits an
electron
Q (MP - MD)c2
b decay
Q (MP - MD 2me)c2
A proton changes into a neutron and emits a
positron
Electron capture a process that competes with b
decay in which a proton in the nucleus captures
an atomic electron and changes into a neutron
with the emission of a neutrino
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Gamma decay
A process in which a nucleus in an excited state
decays to a lower energy state of the same
isotope by the emission of a photon. We saw an
example of this earlier in the decay of
223Ra Internal conversion is a competing
process especially for lower-lying energy
states, in which the excitation energy of the
state is transferred to an orbital electron which
is ejected from the atom. The ejected electron
is observed to have a kinetic energy equal to the
nuclear transition energy minus the electrons
atomic binding energy
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The compound nucleus.
X(x,y)Y
Q (mx mX -my -mY) c2
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Some examples
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Cross section for the 10Be ? reaction the
resonances correspond to excited states of 14N
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The same kind of information about excited states
of 14N can be obtained by inelastic proton
scattering
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The effect of resonances on the cross section
(here the neutron-capture on silver, can be quite
dramatic. The dashed line is an extension of
the1/v behavior expected in the absence of
resonances
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Before we leave this, look at the flipped version
of the plot of B/A vs A that we saw earlier.
Note that the rest energy per nucleon is less for
intermediate mass nuclei than for very heavy or
light ones.the key to fission
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The Nuclear Force About a hundred times stronger
than the Coulomb force Very short rangegoes to
zero beyond about 3 fm Charge independentdoes
not matter if the particles are protons or
neutrons Saturatedis constant at about 8
MeV/nucleon above A20 or so Depends on the spin
orientation of the nucleons Suspected to be an
exchange force in which the attraction is due to
an exchange of pions
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The nuclear shell model It is an
independent-particle model, similar to that used
for assigning energy states to atomic electrons,
but opne that makes use of a strong spin-orbit
coupling for each nucleon. It accounts for the
shell-like structure of protons and neutrons and
explains the magic numbers
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Summary

• Its been a quick trip, and I hope a
  not-too-boring one. I tried to touch on a number
  of topics
• Ground-state properties of the nucleus
• Radioactivity
• Nuclear reactions
• Decay modes of an excited nucleus, and
• The nuclear force
• I hope that there was something in there that you
  found interesting.
• If questions arise during the school year, please
  call (574.631.8591) or drop me a note at
  ahyder_at_nd.edu
• Tony Hyder

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• Bonus
• Who is this?
• Whats wrong with this picture?